Mobile communication systems typified by e.g. GSM (Global System for Mobile Communications) and GPRS (General Packet Radio Service) have been used almost all over the world, and are expected to continue to be used in the future. Mobile communication systems such as GSM and GPRS require controlling the transmitted power of a mobile data terminal according to the communication distance between a base station and the mobile data terminal. The transmitted power control is implemented by controlling the power gain of an RF power amplifier circuit incorporated in a mobile data terminal by use of a control voltage.
The Japanese Unexamined Patent Publication No. JP-A-2004-193846 describes a means to change the gate bias voltage of a transistor of each of multiple amplification stages of an RF power amplifier circuit of a radio communication system according to an output-power-control voltage.
Further, the Japanese Unexamined Patent Publication No. JP-A-2006-270670 likewise describes a means to change the gate bias voltage of a transistor of each of multiple amplification stages of an RF power amplifier circuit of a radio communication system according to an output-power-control voltage. A biasing circuit operable to generate a gate bias voltage for an N-channel MOS FET of each of the multiple amplification stages includes a biasing transistor which is connected with the corresponding N-channel MOS FET to form a current mirror. The gate and drain of the biasing transistor are connected together, whereby the biasing transistor is diode-connected. Each biasing transistor is supplied with a bias current through the drain of a corresponding P-channel MOS FET. The sources of the P-channel MOS FETs are connected to a power source, and the gates of the P-channel MOS FETs are controlled by a voltage from an output terminal of an operational amplifier. The bias currents from the P-channel MOS FETs are controlled by a voltage drop by a resistance connected with a non-inverting input of the operational amplifier. The voltage drop by the resistance is controlled by an output-power-control voltage, which is supplied to an inverting input of the operational amplifier.
In the Japanese Unexamined Patent Publication No. JP-A-2005-217558, it is described, as a means for increasing the power efficiency in a low power mode, to switch stepwise the gate bias voltage of a transistor of the final stage amplifier of multiple stages' amplifiers of an RF power amplifier, which is supplied with a power source controlled according to an output-level-control signal, according to an output-level-control signal thereby to make the bias voltage in the low power mode lower than that in the high power mode.
In the RF power amplifier circuit as described in the JP-A-2006-270670, the drain current of the P-channel MOS FET for bias current setting is supplied to an input terminal of a square-root circuit; an output current resulting from the square root conversion, which arises from an output terminal of the square-root circuit, is supplied to the resistance for bias current setting. Hence, the bias current of the transistor of each of multiple amplification stages of an RF power amplifier circuit takes a value in proportion to the square of the output-power-control voltage, and therefore a sharp change of the transmitted power in response to a change of the output-power-control voltage can be lightened.